Swing excitation of magnetic fields in trailing spiral galaxies?

Swing-excited dynamo-induced magnetic fields in spiral galaxies are studied. We investigate the phenomenon of swing excitation with linear and nonlinear 3D numerical simulations of the galactic dynamo. The model includes differential rotation, axisymmetric and isotropic α-effect and uniform eddy diffusivity. The nonaxisymmetry is introduced via large-scale radial and azimuthal velocity components associated with a spiral density-wave perturbation. In a first step we present a linear analysis in order to get insight in basic properties of the swing-excitation phenomenon. Both one and two-armed spiral galaxies are investigated. We find enlarged growth rates for the magnetic energy connected with the parametric resonance conditionΩp ≈ 2ωm (Ωp pattern speed, ωm magnetic drift rate), but only if the amplitude of the perturbation exceeds 10 km s−1. The resonance behavior also appears in the nonlinear regime. The solutions are a mixture of several magnetic field modes. For a two-armed spiral the even modes (m = 0, 2,...) are preferred. The contribution of higher magnetic field modes to the solution is largest if the parametric resonance condition is fulfilled. The field geometry depends strongly on the pattern speed Ωp, the excitation is weakest at the radius where the differentially rotating gas and the spiral pattern are corotating.